ISSN:0018-019X    

DOI:10.1002/hlca.19930760102

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

ISSN:0018-019X    

DOI:10.1002/hlca.19930760103

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractThe benzyl‐protected glycosyl acetates1,6,11, and15react with MeOPPh2under catalysis by TMSOTf to yield diastereoselectively the glycosylphosphine oxides2,3,8,12,13, and16, with a strong preference for the 1,2‐cis‐configurated anomers. Hydrogenolysis of the major products gave the crystalline, unprotected phosphine oxides4,9,14, and17, of which4was transformed in to the acetate5, and9into the benzoate10. The benzylated phosphine oxides4,8,12, and16were reduced with Cl3SiH in the presence of a tertiary amine to form the phosphines18,21,24, and26, which were transformed into the phosphine sulfides19,22,25, and27. Moreover,18and21, were characterized as the borane adducts20, and23. The structure of the (arabinofuranosyl)phosphine oxide12, the corresponding sulfide25, and of the borane complex20were established by X‐ray analysis. According to NMR spectroscopy, the equatorial pyranosylphosphine oxide8, the sulfide22, and the borane complex23adopt a4C1conformation. The axial phosphine oxide2is a flattened4C1, the sulfide19exists as aB2,5, and the borane complex20is a flattened4C1in the solid sate and aB2,5in solution. Thus, the conformational behavior of these α‐D‐glucopyranose derivatives reflects the steric requirement of the P

ISSN:0018-019X    

DOI:10.1002/hlca.19930760104

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractLabeling experiments provide evidence that the Fe1‐ and CO1‐mediated losses of H2and 2 H2from tetraline are extremely specific in that both reactions follow a clearsyn‐1,2‐elimination involving C(1)/C(2) and C(3)/C(4), respectively. In the course of the multi‐step reaction, the metal ions do not move from one side of the π‐surface to the other. Independent experiments confirm that the kinetic isotope effect (KIE) associated with the loss of the first H2molecule is indeed larger than the KIE for the elimination of the secon

ISSN:0018-019X    

DOI:10.1002/hlca.19930760105

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractThe highly active immunosuppressive antibiotic rapamycin is a specific inhibitor of a signal transduction pathway that results in exocytosis and transcription. We report the results of the homo‐ and heteronuclear NMR experiments of rapamycin in DMSO leading to a complete assignment of the1H‐ and13C‐NMR signals. With exception of one CH2group, all diastereotopic assignments could be achieved using heteronuclear long‐range coupling constants. Restrained molecular‐dynamics simulation in the same solvent as the NMR experiments led to a well defined conformation of the rapamycin molecule in solution. Differences between the solution and crystal structures are

ISSN:0018-019X    

DOI:10.1002/hlca.19930760106

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractFormaldehyde reacts with substituted nitrosobenzenes giving the correspondingN‐phenylhydroxamic acids. A mechanism involving three sequential steps in this reaction is proposed. The first step is the nucleophilic attack of the nitroso group on the carbonyl group which leads to the formation of the unstable tetrahedral zwitterionic intermediate. This step is followed by the proton transfer to the zwitterionic intermediate to form more stable nitrosocarbinolic cation intermediate, which in the subsequent step undergoes the rate‐controlling elimination of proton from the C‐atom of nitrosocarbinolic group, leading to the final product, hydroxamic acid. The first and the second step appear to be reversible. The experimental evidence obtained, which is the basis for such a description of the investigated reaction, includes: (a) the order of reactivity of substituted nitrosobenzenes, as demonstrated by the plot of logkobsvs. σHammettparameters with slope of −1.74; (b) the observation of a general‐acid catalysis; (c) the observation of the inverse solvent deuterium isotope effect ofca. 1.8 in the reaction; (d) the observation of kinetic primary deuterium isotope effect ofca. 8 related to the ‘water’ reaction in the reactions of formaldehyde with substituted nitrosobenzenes; (e) the observation of general‐base catalysis in the reaction; (f) the observation of the kinetic primary deuterium isotope effect ofca. 2.1 for the acetate‐ion

ISSN:0018-019X    

DOI:10.1002/hlca.19930760107

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

Abstract(o‐Hydroxyphenyl)methylphosphonic acids are readily obtained fromo‐(bromomethyl)‐ oro‐(hydroxymethyl)phenols and trialkyl phosphites. Subsequent hydrolysis leads to the corresponding phosphonic acids. For a series of such compounds, the pKavalues have been determined by potentiometry. Their dependence on additional substituents in the aromatic ring is discussed in terms of electronic and steric

ISSN:0018-019X    

DOI:10.1002/hlca.19930760108

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractPrelactones C (3) B (8) were isolated from the concanamycin‐producingStreptomyces sp.(strain Gö 22/15) and bafilomycin‐producingStreptomyces griseus(strain Tü 2599), respectively, by chemical screening methods. The constitution and relative configuration of3and8were established by one‐ and two‐dimensional NMR methods. The absolute configuration of3was determined using theHelmchenmethod and that of8by CD spectra. The structural properties and absolute configuration of these new δ‐lactones reveal strong similarities to the hemiacetal portion of the corresponding macrolide antibiotics. Their possible role in the early polyketide formation of unusual macrolactones

ISSN:0018-019X    

DOI:10.1002/hlca.19930760109

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractHigh‐yield chemical syntheses of phosphoramidite building blocks of the four arabinonucleosides aUrd (1), aCyd (2), aAdo (3), and aGuo (4), suitable for (3′–5′)oligoarabinonucleotide synthesis are described. The problem of 2′‐hydroxy group protection was solved by introduction of the versatile 2‐(4‐nitrophenyl)ethoxycarbonyl (npeoc) residue, which was also used for agl

ISSN:0018-019X    

DOI:10.1002/hlca.19930760110

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractIt is shown that azulene (1) and dimethyl acetylenedicarboxylate (ADM) in a fourfold molar excess react at 200° in decalin to yield, beside the known heptalene‐ (5) and azulene‐1,2‐dicarboxylates (6), in an amount of 1.6% tetramethyl (1RS,2RS,5SR,8RS)‐tetracyclo[6.2.2.22,501,5]tetradeca‐3,6,9,11,13‐pentaene‐3,4,9,10‐tetracarboxylate(‘anti’‐7) as a result of a SHOMO (azulene)/LUMO(ADM)‐controlled addition of ADM to the seven‐membered ring of1followed by aDiels‐Alderreaction of the so formed tricyclic intermediate16(cf. Scheme 3) with a second molecule of ADM. The structure of ‘anti’‐7 was confirmed by an X‐ray diffraction analysis. Similarly, the thermal reaction of 5,7‐dimehtylazulene (3) with excess ADM in decalin at 120° led to the formation ofca. 1% of ‘anti’‐12, the 7,12‐dimethyl derivative of‘anti’‐7, beside of the corresponding heptalene‐10and azulene‐1,2‐dicaboxylated (cf Scheme 2). The introduction of Me groups at C(1)and C(3)of azulene (1) and its 5,7‐dimethyl derivative3strongly enhance the thermal formation of the corresponding tetracyclic compound. Thus, 1,3‐dimethylazulene (2) in the presence of a sevenfold molar excess of ADM at 200° yielded 20% of ‘anti’‐9beside an equal amount of dimethyl 3‐mehtylazulene‐1,2‐dicarboxylate (8;cf. Scheme 1), and 1,3,5,7‐tetramethylazulene (4) with a fourfold molar excess of ADM AT 200° gave a yield of 37% of‘anti’‐15beside small

ISSN:0018-019X    

DOI:10.1002/hlca.19930760111

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY